High-pressure transducers play several important roles in a variety of industries. The most demanding application for precision high-pressure transducers may be found in the oil and gas production industry. The oil and gas production capacity of a reservoir is determined from models using the precision measurement of pressure during and after product is vented through a standard orifice. Monitors for reservoir pressure operate in a “downhole” environment with pressures up to 200 MPa (25,000 psig) and temperatures up to 300° C. This measurement is critical to energy production companies since it directly influences the ability to finance operations.
Additional applications for high-pressure transducers include monitoring water jet cutting equipment, plastic extrusion, and hydroforming processes. Operating pressures for water jet cutting machines may exceed 413 MPa (60,000 psig). Current pressure monitors utilize strain gauge-based devices. The accuracy of these gauges rarely exceeds 0.01%. Hydroforming is a machining technique that uses high pressure to force the work material onto a die. Proper operation of the hydroformer requires high dynamic monitoring of extreme pressures.
Quartz pressure transducers, which have earned their reputation in high standard pressure sensing, have been widely used for some of those crucial applications. The high accuracy and stability of the quartz thickness shear mode resonance (TSMR) have been used as the high quality frequency standard sensing technique, along with surface acoustic wave (SAW) devices1. The advantages of quartz pressure sensors are: good pressure range (up to 280 MPa), high resolution (1 ppm [parts per million] or 0.0001%), lower susceptibility to environmental parameters such as temperature (with special cutting), and long-term stability within a protected environment. The resolution and accuracy come from the high resonance quality (Q factor). The quartz Q factor can be as high as 4000, which needs to be carefully preserved in a hermetically sealed, evacuated environment.
However, there are several problems with quartz pressure sensing technologies that trigger motivation to seek new methods to address these needs. Quartz pressure transducers appear to be limited for applications by higher pressure due to mechanical “twinning” of the pressure sensitive crystal element. “Twinning” is the reversal of the piezoelectric polarity under stress. For a crystal oscillator, the onset of twinning stops the crystal oscillation. For applications above 280 MPa, new materials and methods must be used.
As new techniques have been introduced in the oil and gas industry, new requirements have pushed quartz pressure transducers to the limits of their capabilities. Horizontal production techniques, which allow the extension of the production zone, mandate the use of smaller pressure transducers. Current transducers are typically larger than one inch in diameter. A target diameter of one-half inch or less is desirable to support horizontal operations; such size reductions are difficult for quartz technology to achieve.
The quartz pressure transducer technology dates from the middle 1960s. Since that time, crystal manufacturing operations have been migrating overseas to minimize labor expenses. Crystal manufacturers that previously provided the special purpose elements used in the quartz sensors as an adjunct to the more volume-oriented frequency control business have been severely and negatively impacted by this migration. The development expense is increased by the reluctance of crystal manufacturers to participate in specialty crystal design given the minimal potential market. The lifetime of quartz pressure transducers was, perhaps, underestimated since many existing tools date from the original production. Given the aging fleet of existing sensors, an opportunity exists to infuse a new and more “manufacturable” technology.
Additionally, the quartz reference and sensors along with the necessary electronics package must operate in harsh environments for long periods of time (e.g., several years) with a minimum of maintenance. The severe environment dictates that extensive efforts must be made to protect the equipment. These measures contribute to the expense of the transducer. Transducer calibration also must be certified for temperature and pressure. The certification process is time consuming and adds to the transducer cost.
In natural quartz, the left and right forms are about evenly distributed, resulting in optically twinned material. The presence of twinning prevents the crystal from being used as a resonator. On the other hand, cultured quartz is mostly right forms of quartz, so that twinning is not a problem. However, under very high pressure, several deformation mechanisms, such as mechanical twinning and creeping, may occur in the crystal lattice structure. These pressure-induced defects cause the failure of those quartz-based pressure sensors. This fact sets the upper limit of quartz pressure sensing to about 140–280 MPa.
Finally, yet importantly, it is almost impossible for the quartz transducer to perform direct sensing without the translation of a force collector such as a bellows or Bourdon tube because quartz has no tolerance to the surface contamination. The introduction of a force collector not only injects hysteresis and noise but also complicates the design.
High quality factor mode (HQM) micro-resonator technology is a unique optical resonance phenomenon with extremely high resonance quality factor (Q factor can be ˜1E10 or 1010), which inherently enables itself in the ultra-high resolution spectroscopy. HQM micro-resonators are mostly made of fused silica or glass material, which does not suffer lattice defects under significant pressure. The pressure sensitivity is similar to or better than quartz while it can maintain elasticity up to 9 GPa.
These micro-resonators can be realized in the form of micro-spheres, micro-cylinders, or even a micro-ring/disk structures embedded in an optical chip. Their diameters typically are as small as 5 μM or up to a millimeter, but most are around 100 μm or less.
The following list summarizes some of the technical merits of HQM microresonator-based pressure sensing technology:                Ultra-high resolution technology        Capability to sustain pressure above 500 MPa        Microscopic sensing element        Completely passive device        Optical interrogation        
Researchers have realized since the days of Lord Rayleigh that dielectric materials can be used as waveguides and optical resonators. One of the famous HQM resonance is the so-called whispering-gallery mode (WGM) resonance of spherical dielectric particles, which was studied in detail nearly sixty years ago. A microsphere is essentially a fused quartz ball (typically on the order of 100 μm more or less in diameter). Since the sphere is generally more optically dense than its surrounding medium, light in the sphere can be internally reflected. Light propagating inside the sphere would then be spatially constrained to travel along the perimeter of a great circle of the sphere (the perimeter of a plane that intersects the sphere with maximum area). The light propagates inside the sphere until it is absorbed or scattered by material imperfections. Light constrained in this way is said to be trapped in a whispering-gallery mode. WGM resonators have been proposed for several applications, such as add/drop filters in optical communication, optical switches, laser cavities, and high-resolution spectrometers.
Excitation and interrogation of the HQM's can be accomplished by evanescent-wave coupling. Input and output coupling can be achieved by overlapping the HQM's evanescent field with that of a prism or eroded/angle-polished/tapered single-mode optical fiber by way of example, but not limitation. FIG. 2 shows a waveguide coupling light into the microring resonator from its left side.
Braginsky et al. pointed out several years ago that the low losses and small electromagnetic mode volumes of HQM make high-Q microresonators attainable. A resonance quality (O) as high as 8E9 has been observed in the laboratory environment and Q up to 1E9 can be preserved in protected environments (such as hermetically sealed boxes) for a long period. Due to the nature of high-Q resonance, the spectral peaks (or nulls) of the resonance spectrum are very narrow, which essentially provides the capability of very narrow-band optical filtering.
The outstanding resonance quality of HQM's can be directly translated to high measurement resolution and stability. The spectral resolution can be derived by the Q factor and amplitude resolution as follow. The full-width at half-maximum (FWHM) can be expressed as
      λ    Q    .If the resonance peak is roughly modeled as a triangular shape, then the spectral resolution can be derived as the multiplication of amplitude resolution and FWHM. For example, the FWHM is 1.55 pm (picometer) for a Q=1E7 resonator at 1550 nm. With 1% amplitude resolution, the spectral resolution will be 0.0015 pm. This number is based on the spectral shift measurement without going into interferometer design.
Fiber Bragg grating (FBG) is one widely used sensing technology used to optically measure pressure or force-induced strain. However, due to its limited Q factor (˜1E4 or lower) it is difficult to provide enough measurement resolution because of its broad spectral feature. Several FBG-based pressure sensing studies have been published. Xu et al. has reported a FBG sensor with 0.22-nm spectral shift under 70-MPa for direct sensing. The pressure sensitivity is about 3 pm/MPa. Other research has used special side-hole FBG and boosted the sensitivity about two times. A FBG-based pressure sensor commercialized by Sabeus has listed resolution of 0.05%, which is consistent with our analysis. With limited pressure sensitivity and spectral resolution, the FBG is less appealing in the high-resolution pressure sensing.
Though FBG technology may not be suitable to for high-resolution pressure sensors, many research and development efforts in temperature compensation and signal interrogation can be translated into HQM resonator technology because of their common nature in spectral domain interrogation and temperature compensation.
Currently, quartz transducers typically employ specialized quartz sensors. A typical quartz pressure transducer would include a reference oscillator, using a temperature- and stress-compensated crystal, a quartz temperature sensor, and a quartz pressure sensor. The quartz temperature sensor provides a temperature measurement independent of pressure so that the pressure measurement may be compensated for temperature variations. The quartz pressure sensor is designed for a specific response to stress applied in a plane determined by a quartz force collector. The pressure sensor is exposed to external pressure while the electronics package, temperature, and reference elements are isolated.
The HQM resonances will not only respond to the strain but also to the temperature. Xu et al. first reported a discrimination technique by using superimposed dual FBGs at 850 and 1300 nm. Afterward, many approaches were reported by using dual FBG with different fiber materials, diameters, and grating types. These methods are all based on the differentiation of strain and temperature effect upon spectral shift. The spectral shift of the two FBG wavelengths (Δλ1,2) can be modeled as follow:
            [                                                  Δλ              1                                                                          Δλ              2                                          ]        =                  [                                                            K                ɛ1                                                                    K                T1                                                                                        K                ɛ2                                                                    K                T2                                                    ]            ⁡              [                                            Δɛ                                                                          Δ                ⁢                                                                  ⁢                T                                                    ]              ,
where Kε1,2 are the strain-response coefficients, KT1,2 are the thermo-response coefficients, Δε is the applied strain due to pressure, and ΔT is the temperature change. The contribution due to pressure-induced stress is introduced through Young's modulus, Poisson ratio, and photoelastic constant, while the contribution from temperature is determined by thermal expansion and thermo-optic coefficients. Therefore, the coefficient ratio
      K    ɛ1        K    ɛ2  should be different from
            K      T1              K      T2        .This discrepancy provides information to solve Δε and ΔT, respectively.
Thus, a need exists for an improved pressure sensing system which has a pressure sensor which can be made smaller in size and capable of accurately measuring higher pressures than the quartz pressure transducers discussed above. It is to such an improved pressure sensing system that the present invention is directed.